Fiber optical system with fiber end face area relationships

ABSTRACT

An optical network having at least one star coupler comprising transmit and receive optical mixers which are respectively optically coupled to transmitters and receivers of a plurality of optical-electrical media converters. Each optical-electrical media converter comprises a respective receiver optically coupled to the receive optical mixer by way of plastic optical fibers and a respective transmitter optically coupled to the transmit optical mixer by way of plastic optical fibers. The output plastic optical fibers attached to an output face of the receive optical mixer have a diameter less than the diameter of the input plastic optical fibers attached to an input face of the receive optical mixer.

BACKGROUND

The technology disclosed herein generally relates to optical networksthat enable communication between electrical components.

Optical networking using plastic optical fibers may provide advantagesover networking using copper or other metal wiring. Categories ofplastic optical fiber include plastic-clad silicon optical fiber,single-core plastic optical fiber, or multi-core plastic optical fiber.Plastic optical fiber networking may have lower installation andmaintenance costs. Moreover, because plastic optical fibers are lighterthan the metal wiring that would be needed to carry an equivalent amountof data, using plastic optical fibers may result in appreciable weightsavings. The weight savings may be significant for networks onboardvehicles, such as aircraft, where the weight savings may result inreduced fuel consumption and lower emissions.

In some scenarios, it is desirable to connect a number of linereplaceable units to each other. For example, a number of linereplaceable units in the forward section of a vehicle (e.g., anaircraft) may need to be connected to a number of line replaceable unitsin the aft section of the vehicle. Connecting each line replaceable unitto every other line replaceable unit could result in an unreasonablylarge number of connections between line replaceable units.Additionally, many of the connections between line replaceable units maybe long, resulting in optical losses. If all of these connections werein the form of copper wires, the resulting space and weight of theconnections could be burdensome for the vehicle. Electrical data buseshave been used to connect line replaceable units. A single optical databus can eliminate some of the weight and size of electrical connectionsbetween line replaceable units. In general, optical communicationfibers, such as glass optical fibers and plastic optical fibers, can belighter and contained in smaller spaces than electrical wiring. However,implementing optical communication systems is not as simple as merelyreplacing all electric wiring with optical fibers.

Plastic optical fibers have high transmission capacity, excellentimmunity to electromagnetic interference-induced noise, light weight,high mechanical strength, and outstanding flexibility. Due to theseproperties, plastic optical fibers are used in data communications, aswell as decoration, illumination, and similar industrial applications.Plastic optical fibers are also larger in diameter as compared to glassoptical fibers. Due to their larger diameters, plastic optical fibershave greater tolerance for fiber misalignment than glass optical fibershave. Because of this large misalignment tolerance, plastic opticalfiber-based networks have lower maintenance and installation costs. Inaerospace platforms, plastic optical fibers also greatly reduce the costof connectors and transceiver components used in an avionics network.

Currently, some optical data bus architectures (e.g., an ARINC 629plastic optical fiber (POF) data bus) employed in aircraft require anindividually packaged optical-electrical media converter for eachchannel. They also require individually packaged passive optical starcouplers. These individually packaged units are interconnected togetherby fully jacketed POF cables.

As used herein, the term “star coupler” comprises one or more devices ofa type that receives a plurality of optical signals at an input face viarespective input optical fibers and outputs respective portions of eachreceived optical signal to each of a plurality of output optical fibersoptically coupled to an output face of the device. Thus each outputoptical fiber receives respective input optical signals from all of theinput optical fibers. It is known to combine two devices of this type toform a star coupler that can be optically coupled to the transmittersand receivers of a plurality of optical-electrical media converters toenable a plurality of electronic components (such as line replaceableunits), which are respectively electrically connected to theoptical-electrical media converters, to communicate with each other.

An existing solution uses dual symmetric star couplers having input andoutput faces optically coupled to 1-mm-diameter plastic optical fibers,which plastic optical fibers are also connected to transmitters andreceivers of respective optical-electrical media converters. In a knowncase, each receiver of an optical-electrical media converter comprises aphotodetector having a diameter less than 1 mm (e.g., 0.4 mm). Becausethe 1-mm-diameter output plastic optical fibers optically coupled to thereceivers are larger than the photodetector, this mismatch produces anoptical coupling loss.

There is a need for a solution that reduces, if not eliminates, opticalcoupling loss due to mismatched sizes of POF end faces andphotodetectors incorporated in receivers of optical-electrical mediaconverters.

SUMMARY

The subject matter disclosed in detail below is directed to an opticalnetwork that enables communication between electrical components such asline replaceable units on an aircraft. The optical network comprises atleast one star coupler comprising a transmit optical mixer and a receiveoptical mixer, which optical mixers are connected to the transmittersand receivers respectively of a plurality of optical-electrical mediaconverters. Each optical-electrical media converter comprises arespective receiver optically coupled to the receive optical mixer byway of output plastic optical fibers and a respective transmitteroptically coupled to the transmit optical mixer by way of input plasticoptical fibers. In accordance with embodiments that will be described inmore detail below, the output plastic optical fibers attached to anoutput face of the receive optical mixer have a diameter less than thediameter of the input plastic optical fibers.

As used herein, the term “transmit optical mixer” means an optical mixerin which the attached input plastic optical fibers are optically coupledto transmitters. As used herein, the term “receive optical mixer” meansan optical mixer in which the attached output plastic optical fibers areoptically coupled to receivers. The term “asymmetric”, as applied tooptical mixers herein, means that the diameter of the input plasticoptical fibers is different than the diameter of the output plasticoptical fibers.

In accordance with the embodiments disclosed herein, the receive opticalmixers are connected to 1-mm-diameter input plastic optical fibers andto smaller-diameter (i.e., less than 1 mm) output plastic optical fibersto enhance receiver sensitivity. As compared to symmetrical receiveoptical mixers connected to 1-mm-diameter plastic optical fibers only,the use of smaller-diameter output plastic optical fibers improves thereceiver sensitivity by better matching to the diameter (in thedisclosed example, 0.4 mm) of the photodetectors integrated into thereceivers.

One aspect of the subject matter disclosed in detail below is a fiberoptical system comprising: a mixing optical fiber having a first endface with a first area and a second end face with the first area; afirst input plastic optical fiber having a first end face with the firstarea and a second end face with a second area less than the first area,the second end face of the first input plastic optical fiber beingattached to a first portion of the first end face of the mixing opticalfiber; a second input plastic optical fiber having a first end face withthe first area and a second end face with a third area less than thefirst area, the second end face of the second input plastic opticalfiber being attached to a second portion of the first end face of themixing optical fiber; and a plurality of output plastic optical fibershaving end faces with a fourth area less than the first area attached tothe second end face of the mixing optical fiber. The sum of the secondand third areas is preferably equal to the first area.

In accordance with some embodiments of the fiber optical systemdescribed in the preceding paragraph, the first input plastic opticalfiber has a first side face that intersects the second end face of thefirst input plastic optical fiber, and the second input plastic opticalfiber has a second side face that intersects the second end face of thesecond input plastic optical fiber. In these embodiments, the systemfurther comprises a layer of index matching epoxy disposed between andbonding confronting portions of the first and second side faces with nometal layer therebetween.

Another aspect of the subject matter disclosed in detail below is afiber optical system comprising: a mixing optical fiber having a firstend face with a first area and a second end face with the first area; acombiner having an end face with a second area attached to the first endface of the mixing optical fiber; a first length of plastic opticalfiber having a cross-sectional area equal to the first area andoptically coupled to the combiner; a second length of plastic opticalfiber having a cross-sectional area equal to the first area andoptically coupled to the combiner; and a plurality of output plasticoptical fibers having end faces with a third area less than the firstarea attached to the second end face of the mixing optical fiber.Preferably the second area is equal to the first area. In accordancewith some embodiments of the fiber optical system described in thepreceding paragraph, the combiner comprises first and second portionshaving respective end faces attached to the first end face of the mixingoptical fiber, the first and second portions of the combiner beingbonded together by a layer of index matching epoxy, the first length ofplastic optical fiber being integrally formed with the first portion ofthe combiner and the second length of plastic optical fiber beingintegrally formed with the second portion of the combiner. In accordancewith other embodiments, the combiner may be a separate opticallytransparent component having two input end faces attached to end facesof respective plastic optical fibers and an output end face attached toan end face of the mixing optical fiber. The mixing optical fiberpreferably comprises a step-index plastic optical fiber.

A further aspect of the subject matter disclosed in detail below is adata communications system comprising: first and second pluralities ofelectrical devices configured for sending and receiving electricalsignals representing data; a first plurality of optical-electrical mediaconverters, each optical-electrical media converter of the firstplurality of optical-electrical media converters comprising a respectivetransmitter that converts electrical signals received from a respectiveone of the first plurality of electrical devices into optical signalsand a respective receiver that converts optical signals into electricalsignals to be sent to the respective one of the first plurality ofelectrical devices; a second plurality of optical-electrical mediaconverters, each optical-electrical media converter of the secondplurality of optical-electrical media converters comprising a respectivetransmitter that converts electrical signals received from a respectiveone of the second plurality of electrical devices into optical signalsand a respective receiver that converts optical signals into electricalsignals to be sent to the respective one of the second plurality ofelectrical devices; a first plurality of input plastic optical fibersrespectively optically coupled to the transmitters of the firstplurality of optical-electrical media converters and having end faceswith a first area; a second plurality of input plastic optical fibersrespectively optically coupled to the transmitters of the secondplurality of optical-electrical media converters and having end faceswith the first area; a first plurality of output plastic optical fibersrespectively optically coupled to the receivers of the first pluralityof optical-electrical media converters and having end faces with asecond area less than the first area; a second plurality of outputplastic optical fibers respectively optically coupled to the receiversof the second plurality of optical-electrical media converters, eachoutput plastic optical fiber having end faces with a third area lessthan the first area; a first optical star coupler comprising a firsttransmit optical mixer having an input face attached to the end faces ofthe first plurality of input plastic optical fibers and having an outputface, a first receive optical mixer having an output face attached tothe end faces of the first plurality of output plastic optical fibers,and a first wrap-around fiber optical path that has a first end faceattached to the output face of the first transmit optical mixer and asecond end face with a fourth area less than the first area attached tothe first receive optical mixer; a second optical star couplercomprising a second transmit optical mixer having an input face attachedto the end faces of the second plurality of input plastic optical fibersand having an output face, a second receive optical mixer having anoutput face attached to the end faces of the second plurality of outputplastic optical fibers, and a second wrap-around fiber optical path thathas a first end face attached to the output face of the second transmitoptical mixer and a second end face with a fifth area less than thefirst area attached to the second receive optical mixer; a first fiberoptical path that has a first end face attached to the output face ofthe first transmit optical mixer and a second end face with a sixth arealess than the first area attached to the second receive optical mixer;and a second fiber optical path that has a first end face attached tothe output face of the second transmit optical mixer and a second endface with a seventh area less than the first area attached to the firstreceive optical mixer. The first receive optical mixer comprises a firstmixing optical fiber having a first end face with the first areaattached to the second end faces of the first wrap-around plasticoptical fiber and the second fiber optical path; and having a second endface with the first area attached to the end faces of the firstplurality of output plastic optical fibers. The second receive opticalmixer comprises a second mixing optical fiber having a first end facewith the first area attached to the second end faces of the secondwrap-around plastic optical fiber and the first fiber optical path; andhaving a second end face with the first area attached to the end facesof the second plurality of output plastic optical fibers. In accordancewith some embodiments, the sum of the fourth and seventh areas and thesum of the fifth and sixth areas are respectively equal to the firstarea, the fourth and seventh areas are equal, and the fifth and sixthareas are not equal. Each of the receivers of the first and secondpluralities of optical-electrical media converters comprises arespective photodetector having an eighth area which is less than thefirst area. In accordance with some embodiments, the first plurality ofelectronic devices are line replaceable units located in a forwardsection of an aircraft and the second plurality of electronic devicesare line replaceable units located in an aft section of the aircraft.

Yet another aspect is a method for installing an optical mixer in anoptical network, comprising: cutting a length of a first plastic opticalfiber to form first and second end faces, each of the first and secondend faces having a first area; shaping an end section of a secondplastic optical fiber having a cross-sectional area equal to the firstarea to form a first side face that intersects and is perpendicular to afirst end face having a second area which is less than the first area;shaping an end section of a third plastic optical fiber having across-sectional area equal to the first area to form a second side facethat intersects and is perpendicular to a second end face having a thirdarea which is less than the first area; bonding the first and secondside faces of the second and third plastic optical fibers together usingindex matching epoxy; bonding the first and second end faces of thesecond and third plastic optical fibers to respective portions of thefirst end face of the length of the first plastic optical fiber usingindex matching epoxy; bonding the end faces of a plurality of fourthplastic optical fibers, each having a fourth area less than the firstarea, to respective portions of the second end face of the first plasticoptical fiber using index matching epoxy; securing the length of thefirst optical fiber, respective portions of the end sections of thesecond and third plastic optical fibers, and a cladding section of theplurality of fourth plastic optical fibers inside a ferrule usingpotting optical epoxy; and connecting the second and third plasticoptical fibers and the plurality of plastic optical fibers to respectiveother plastic optical fibers of the optical network. Preferably, the sumof the second and third areas is equal to the first area.

The optical networks disclosed herein are designed to enhance theoptical link budget of the optical system and enable the optical systemto achieve a target end-of-life optical power margin as required for theparticular installation. The proposed design is low cost andmanufacturable using commercial-of-the-shelf plastic optical fibercomponents and without using high-temperature fusing processes.

Other aspects of asymmetric receive optical mixers for use in opticalnetworks are disclosed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, functions and advantages discussed in the precedingsection can be achieved independently in various embodiments or may becombined in yet other embodiments. Various embodiments will behereinafter described with reference to drawings for the purpose ofillustrating the above-described and other aspects. None of the diagramsbriefly described in this section are drawn to scale and the relativethicknesses of layers depicted in those diagrams does not preciselyreflect the actual thicknesses.

FIG. 1 is a diagram depicting an aircraft with a number of linereplaceable units connected via a plastic fiber optical network havingtwo symmetrical optical star couplers.

FIGS. 2A and 2B are diagrams representing isometric and side viewsrespectively of a tapered optical mixer in accordance with oneembodiment.

FIGS. 3A, 3B and 3C are diagrams representing first end, side and secondend views respectively of a tapered optical mixer of the type depictedin FIGS. 2A and 2B connected to plastic optical fibers at both ends.

FIG. 4 is a diagram representing an isometric view of a portion of anoptical network comprising a pair of tapered optical mixers which areoptically coupled to each other.

FIG. 5 is a schematic representation of an optical network that includestwo pairs of tapered optical mixers in accordance with one embodiment.

FIG. 6 is a diagram representing a sectional view of an optical couplingof two optical fibers using a connector.

FIG. 7 is a diagram showing an optical network configuration inaccordance with one embodiment with five connector breaks between theforward and aft star couplers.

FIG. 8 is a diagram showing an optical network configuration inaccordance with another embodiment with six connector breaks between theforward and aft star couplers.

FIG. 9 is a diagram showing one end of a 1-mm-diameter plastic opticalfiber optically coupled to a 0.4-mm-diameter photodetector of a receiverby way of a ball lens soldered to a cap.

FIG. 10A is a diagram showing a tapered receive optical mixer of aforward star coupler, which receive optical mixer has a pair of1-mm-diameter input plastic optical fibers which are attached to itsinput face and a plurality of output plastic optical fibers which areattached to its output face.

FIG. 10B is a diagram showing a tapered receive optical mixer of an aftstar coupler, which receiving optical mixer has two 1-mm-diameter inputplastic optical fibers which are attached to its input face and four1-mm-diameter output plastic optical fibers which are attached to itsoutput face.

FIG. 11 is a diagram showing a forward receive optical mixer of aforward star coupler in accordance with one embodiment, which forwardreceive optical mixer comprises a 1-mm-diameter step-index plasticoptical fiber having a pair of symmetric D-shaped end faces ofrespective input plastic optical fibers attached to its input end faceand having end faces of a plurality of 190-μm-diameter output plasticoptical fibers attached to its output end face.

FIG. 12 is a diagram representing an isometric view of two plasticoptical fibers converging to form a combiner in the optical mixerdepicted in FIG. 11.

FIG. 12A is a magnified view of the end portion of the combiner depictedin FIG. 12.

FIG. 12B is a diagram showing the semicircular end faces of the plasticoptical fibers depicted in FIG. 12.

FIG. 13 is a diagram showing the shape of end faces of the plurality of190-μm-diameter output plastic optical fibers depicted in FIG. 11.

FIG. 14 is a diagram showing the true (not idealized) shapes of endfaces of a plurality of 190-μm-diameter output plastic optical fibers ofa commercially available multi-core plastic optical fiber bundle whichcan be attached to the output end face of the 1-mm-diameter step-indexplastic optical fiber depicted in FIG. 11.

FIG. 15 is a diagram showing a multi-core plastic optical fiber bundle,a portion of which has been separated into individual plastic opticalfibers by removal of the outer cladding of the fiber bundle.

FIG. 16 is a diagram showing the shape of end faces of a plurality of175-μm-diameter output plastic optical fibers which can be attached tothe output end face of the 1-mm-diameter step-index plastic opticalfiber depicted in FIG. 11 in accordance with a second embodiment.

FIG. 17 is a diagram showing the shape of end faces of a plurality of175-μm-diameter output plastic optical fibers which can be attached tothe output end face of the 1-mm-diameter step-index plastic opticalfiber depicted in FIG. 11 in accordance with a third embodiment.

FIG. 18 is a diagram showing an aft receive optical mixer of an aft starcoupler in accordance with one embodiment, which aft receive opticalmixer comprises a 1-mm-diameter step-index plastic optical fiber havingan input end face to which two asymmetric end faces of respective inputplastic optical fibers are attached and an output end face to which theend faces of four 400-μm-diameter output plastic optical fibers areattached.

FIG. 19 is a diagram showing the two asymmetric end faces of the inputplastic optical fibers depicted in FIG. 18.

FIG. 20 is a diagram showing the end faces of the four 400-μm-diameteroutput plastic optical fibers depicted in FIG. 18.

Reference will hereinafter be made to the drawings in which similarelements in different drawings bear the same reference numerals.

DETAILED DESCRIPTION

Illustrative embodiments of optical networks are described in somedetail below. However, not all features of an actual implementation aredescribed in this specification. A person skilled in the art willappreciate that in the development of any such actual embodiment,numerous implementation-specific decisions must be made to achieve thedeveloper's specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

Various embodiments of a fiber optical network for enabling opticalcommunication between line replaceable units on an aircraft will bedescribed in detail below for the purpose of illustration. However,implementation of the fiber optical networks disclosed herein is notlimited solely to the environment of an aircraft, but rather may beutilized in fiber optical networks onboard other types of vehicles orfiber optic networks.

It is known to interconnect line replaceable units on an aircraft usinga fiber optical system comprising dual symmetrical star couplers. Insome cases, the line replaceable units are connected to optical starcouplers via plastic optical fibers. In this manner, the signals sent byeach of the line replaceable units are received by all of the other linereplaceable units. Some of the line replaceable units are separated byrelatively long distances.

FIG. 1 depicts an aircraft 400 having a number of line replaceable units401 onboard. For ease of depiction, not all of the line replaceableunits 401 have been labeled. The aircraft vehicle includes an opticalnetwork that enables the line replaceable units 401 to communicate witheach other. In accordance with the embodiment depicted in FIG. 1, theoptical network comprises a forward star coupler 410 disposed in aforward section of the aircraft 400 and an aft star coupler 420 disposedin an aft section of the aircraft 400. The optical network furthercomprises the following: (a) plastic optical fiber transmission lines411 from each media converter of the line replaceable units 401 in theforward section of the aircraft 400 to the forward star coupler 410; (b)plastic optical fiber receiving lines 412 connecting the forward starcoupler 410 back to each media converter of the line replaceable units401 in the forward section of the aircraft 400; (c) plastic opticalfiber transmission lines 421 from each media converter of the linereplaceable units 401 in the aft section of the aircraft 400 to the aftstar coupler 420; (d) plastic optical fiber receiving lines 422connecting the aft star coupler 420 back to each media converter of theline replaceable units 401 in the aft section of the aircraft 400; (e) afirst long plastic optical fiber transmission line 431 connecting theforward star coupler 410 to the aft star coupler 420; and (f) a secondlong plastic optical fiber transmission line 432 connecting the forwardstar coupler 410 to the aft star coupler 420.

An optical fiber is a cylindrical dielectric waveguide that transmitslight along its axis. The fiber consists of a transparent coresurrounded by a transparent cladding layer (hereinafter “cladding”),both of which are made of dielectric materials. Light is kept in thecore by the phenomenon of total internal reflection. To confine theoptical signal in the core, the refractive index of the core is greaterthan that of the cladding. The boundary between the core and claddingmay either be abrupt, as in step-index fiber, or gradual, as ingraded-index fiber. Although optical fibers can be made of glass orplastic, this disclosure is directed to systems that employ plasticoptical fibers.

In accordance with the embodiments disclosed herein, the forward starcoupler 410 and the aft star coupler 420 each comprise a respective pairof tapered optical mixers. FIGS. 2A and 2B are diagrams representingisometric and side views respectively of an optical mixer 610 inaccordance with one embodiment. The optical mixer 610 has a first face611 and a second face 612. The size 621 of the first face 611 can bebased on a number of optical fibers to be connected to the first face611. The size 622 of the second face 612 can be based on a number ofoptical fibers to be connected to the second face 612. If the number ofoptical fibers to be connected to the first face 611 is different thanthe number of optical fibers to be connected to the second face 612,then the size of the first face 611 and the size of the second face 612can be different, thereby giving the optical mixer 610 a tapered shape.The length 623 of the optical mixer 610 can be based on the sizes 621and 622 of the faces 611 and 612. Each of the faces 611 and 612 can besubstantially centered about an axis 624 that is substantiallyperpendicular to each of the two faces 611 and 612. The mixing rod 610is preferably made of a material that has an index of refraction equalto the index of refraction of the plastic material of the plasticoptical fibers connected to the optical mixer 610.

FIGS. 3A, 3B and 3C are diagrams representing first end, side and secondend views respectively of a tapered optical mixer 610 of the typedepicted in FIGS. 2A and 2B connected to plastic optical fibers at bothends. More specifically, a first set of optical fibers 640 are attachedto a first face 611 of the optical mixer 610 and a second set of opticalfibers 650 attached to a second face 612 of the optical mixer 610. Thesizes of the faces 611 and 612 and the length of the optical mixer 610can be determined such that light entering from any one of the first setof optical fibers 640 will be distributed substantially uniformly acrossthe second face 612 while light entering from any one of the second setof optical fibers 650 will be distributed substantially uniformly acrossthe first face 611. In this manner, when an optical signal enters oneface of the optical mixer 610 from an optical fiber, the same opticalsignal is transmitted to all of the optical fibers attached to theopposite face of the optical mixer 610.

In the example shown in FIG. 3A, the number of optical fibers 640 isnineteen; in the example shown in FIG. 3C, the number of optical fibers650 is four. However, typically the number of optical fibers 640 canvary from seven to forty, while the number of optical fibers 650 canvary from two to four.

The first and second sets of optical fibers 640 and 650 can be alignedto the respective faces 611 and 612 of the optical mixer 630 to optimizecoupling. After alignment, the first and second sets of optical fibers640 and 650 can be attached to the respective faces 611 and 612 of theoptical mixer 610 with an index-matching ultraviolet light-curableadhesive. The completed assembly of optical fibers 640 and 650 andoptical mixer 610 can be packaged in a protective housing. Connectorscan be used to mate with the optical fibers inside the packagedprotective housing with external optical fibers.

FIG. 4 is a diagram representing an isometric view of a portion of anoptical network comprising a pair of tapered optical mixers 710 and 720which are optically coupled to each other by an optical fiber 730.Optical mixer 710 has a first face 711 with a first set of opticalfibers 712 attached thereto and a second face 713 with optical fibers714 and 730 attached thereto. Similarly, optical mixer 720 has a firstface 721 with a second set of optical fibers 722 attached thereto and asecond face 723 with optical fibers 724 and 730 attached thereto. Theoptical mixer 710 can uniformly distribute light from optical signalsreceived from the first set of optical fibers 712 across the second face713 such that the optical signals entering optical fibers 714 and 730are a combination of all of the optical signals received from the firstset of optical fibers 712. In addition, the optical fiber 714 can beconnected to one of another pair of optical mixers (not shown). Inaddition, the optical fiber 730 can carry the combination of opticalsignals received from the first set of optical fibers 712 to the secondface 723 of optical mixer 720. Optical fiber 724 can also carry anoptical signal from one of the other pair of optical mixers (not shown).The optical mixer 720 can uniformly distribute light from the opticalsignals received from optical fibers 724 and 730 across the first face721 such that optical signals entering the second set of optical fibers722 are a combination of all of the optical signals received fromoptical fibers 724 and 730.

FIG. 5 is a schematic representation of an optical network 800 thatincludes four optical mixers 812, 813, 822 and 823 in accordance withone embodiment. The optical network 800 comprises a first plurality ofoptical-electrical media converters 811-1 to 811-N (i.e., the number ofoptical-electrical media converters in the first plurality is N) whichare: (a) respectively electrically coupled to a first plurality of Nline replaceable units 810-1 to 810-N; (b) optically coupled to opticalmixer 812 by way of plastic optical fibers 814; and (c) opticallycoupled to optical mixer 813 by way of plastic optical fibers 815. Eachoptical-electrical media converter of the first plurality comprises: (a)a respective transmitter (not shown in FIG. 5) that has a laser forconverting electrical signals received from a respective linereplaceable unit into optical signals to be sent to optical mixer 812;and (b) a respective receiver (not shown in FIG. 5) that has aphotodetector that converts optical signals received from optical mixer813 into electrical signals to be sent to a respective line replaceableunit.

The optical network 800 further comprises a second plurality of Moptical-electrical media converters 821-1 to 821-M (i.e., the number ofoptical-electrical media converters in the second plurality is M) whichare: (a) respectively electrically coupled to a second plurality of Mline replaceable units 820-1 to 820-M; (b) optically coupled to opticalmixer 822 by way of plastic optical fibers 824; and (c) opticallycoupled to optical mixer 823 by way of plastic optical fibers 825. Eachoptical-electrical media converter of the second plurality comprises:(a) a respective transmitter (not shown in FIG. 5) that has a laser forconverting electrical signals received from a respective linereplaceable unit into optical signals to be sent to optical mixer 822;and (b) a respective receiver (not shown in FIG. 5) that has aphotodetector for converting optical signals received from optical mixer823 into electrical signals to be sent to a respective line replaceableunit.

The optical network 800 depicted in FIG. 5 further comprises opticalfibers 831, 832, 833 and 834. The optical fiber 831 is connected toenable the propagation of optical signals from the smaller end ofoptical mixer 812 to the smaller end of optical mixer 813. The opticalfiber 832 is connected to enable the propagation of optical signals fromthe smaller end of optical mixer 812 to the smaller end of optical mixer823. The optical fiber 833 is connected to enable the propagation ofoptical signals from the smaller end of optical mixer 822 to the smallerend of optical mixer 813. The optical fiber 834 is connected to enablethe propagation of optical signals from the smaller end of optical mixer822 to the smaller end of optical mixer 823. In accordance with theoptical network 800 depicted in FIG. 5, each signal sent by any one ofline replaceable units is received by all of the other line replaceableunits.

In the embodiment depicted in FIG. 5, the first and second opticalmixers 812 and 813 are configured to be connected to N optical fibers onone end and to two optical fibers on the other end. Such optical mixerscan be referred to as 2×N optical mixers. The third and fourth opticalmixers 822 and 823 are configured to be connected to M optical fibers onone end and to two optical fibers on the other end. Such optical mixerscan be referred to as 2×M optical mixers.

In one example, an electrical signal is sent by line replaceable unit810-1 to optical-electrical media converter 811-1, which converts theelectrical signal into an optical signal that is sent to optical mixer812 via one of the optical fibers 814. From the optical mixer 812, theoptical signal is sent to optical mixers 813 and 823 via optical fibers831 and 832 respectively. The second optical mixer 813 receives theoptical signal via optical fiber 831 and sends that optical signal alongeach of the optical fibers 815 to the first plurality ofoptical-electrical media converters 811-1 to 811-N. Those opticalsignals are converted into electrical signals and sent to respectiveones of the first plurality of line replaceable units 810-1 to 810-N. Inthe meantime, the fourth optical mixer 823 receives the optical signalfrom optical mixer 812 via optical fiber 832 and sends that opticalsignal along each of the optical fibers 825 to the second plurality ofoptical-electrical media converters 821-1 to 821-M. The optical signalsare converted into electrical signals and sent to respective ones of thesecond plurality of line replaceable units 820-1 to 820-M.

Due to the long length of some of the fiber optical paths connecting theforward and aft star couplers, it is common to use connectors tooptically couple a plurality of shorter-length plastic optical fibers inseries. FIG. 6 is a sectional view depicting an optical coupling of oneend of a first fiber optic device 8 a to one end of a second fiber opticdevice 8 b using a connector 6. The first fiber optic device 8 acomprises a plastic optical fiber 2 b surrounded by a ferrule 4 b madeof metal (e.g., stainless steel or aluminum) or ceramic, while thesecond fiber optic device 8 b comprises a plastic optical fiber 2 asurrounded by a ferrule 4 a made of metal or ceramic. It is well knownthat each plastic optical fiber depicted in FIG. 6 (and other drawings)comprises a polymeric core (e.g., made of PMMA) surrounded byfluorinated polymeric cladding. In the example depicted in FIG. 6, theplastic optical fibers 2 a and 2 b have substantially the same diameter.Therefore, rays of light (represented by dashed arrows) propagating fromleft to right (as seen in FIG. 6) along plastic optical fiber 2 a canpass into plastic optical fiber 2 b with a small optical coupling loss.

Many different types of optical fiber connectors exist and arecommercially available. Accordingly, FIG. 6 does not seek to depict anyparticular configuration or type of optical fiber connector, but rathersimply represents (in section) a generic connector as a generallycircular cylindrical structure. In addition, it is well known that someconnectors include springs and associated structure for pushing the endsof two fiber optic devices into contact with each other. Such springsand associated structure are also not shown in FIG. 6.

The connector's optical coupling loss depends on the quality of theconfronting (in this example, also abutting) end faces of the plasticoptical fibers (POF) 2 a and 2 b. A poor POF end face can introduce anadditional optical loss per connector. The provision of smooth POF endfaces is important to reduce the connector's optical coupling loss foravionics POF networks where the optical link budget is very tight due torelatively long POF lengths.

During the movements of a vehicle such as an aircraft, vibrations occurin various components at various vibration amplitudes and frequencies.In cases where two components are in contact, vibrations can cause thosecomponents to rub against each other. In cases where the two componentsare made of plastic, the rubbing surfaces of the two components maybecome scratched or develop other defects. To avoid such damage, it isdesirable to provide a fiber optic system in which an air gap is presentbetween the confronting end faces of the plastic optical fibers 2 a and2 b seen in FIG. 6. This can be accomplished by polishing the end facesof the fiber optic devices 8 a and 8 b so that the end faces of theferrules 4 a and 4 b abut while the end faces of the plastic opticalfibers 2 a and 2 b are separated by an air gap, as disclosed in U.S.patent application Ser. No. 15/161,552.

FIG. 7 is a diagram showing a configuration of an optical network inaccordance with one embodiment with five connector breaks in each of twofiber optical paths 30 and 40 connecting a forward star coupler 10 to anaft star coupler 20. The forward star coupler 10 comprises a 24×2transmit optical mixer 12 and a 2×24 receive optical mixer 14. The aftstar coupler 20 comprises a 4×2 transmit optical mixer 22 and a 2×4receive optical mixer 24. The optical mixers are made of opticallytransparent material.

Still referring to FIG. 7, an input face of the 24×2 transmit opticalmixer 12 is connected to respective transmitters Tx1-Tx19 of a pluralityof transmitters 16 by respective plastic optical fibers 36, while anoutput face of the 2×24 receive optical mixer 14 is connected torespective receivers Rx1-Rx19 of a plurality of receivers 18 byrespective plastic optical fibers 38. Each the pluralities of receivers18 may be a monolithic receiver integrated circuit (IC) chip packagedinside a metal cap (further described below with reference to FIG. 9).The transmitters 16 and the receivers 18 are paired in respectiveforward optical-electrical media converters. For example, transmitterTx1 and receiver Rx1 are incorporated in a first forwardoptical-electrical media converter electrically coupled to a firstforward line replaceable unit (not shown in FIG. 7); transmitter Tx2 andreceiver Rx2 are incorporated in a second forward optical-electricalmedia converter electrically coupled to a second forward linereplaceable unit (not shown in FIG. 7); and so forth. The nineteen pairsof transmitters/receivers (Tx1-Tx19/Rx1-Rx19) of the forwardoptical-electrical media converters form nineteen channels, eighteenactive channels and one spare, each channel being coupled to arespective line replaceable unit disposed in the forward section of theaircraft.

Similarly, an input face of the 4×2 transmit optical mixer 22 isconnected to respective transmitters Tx1-Tx4 of a plurality oftransmitters 26 by respective plastic optical fibers 46, while an outputface of the 2×4 receive optical mixer 24 is connected to respectivereceivers Rx1-Rx4 of a plurality of receivers 28 by respective plasticoptical fibers 48. The transmitters 26 and the receivers 28 are pairedin respective aft optical-electrical media converters. For example,transmitter Tx1 and receiver Rx1 are incorporated in a first aftoptical-electrical media converter electrically coupled to a first aftline replaceable unit (not shown in FIG. 7); transmitter Tx2 andreceiver Rx2 are incorporated in a second aft optical-electrical mediaconverter electrically coupled to a second aft line replaceable unit(not shown in FIG. 7); and so forth. The four pairs oftransmitters/receivers (Tx1-Tx4/Rx1-Rx4) of the aft optical-electricalmedia converters form four channels, three active channels and onespare, each channel being coupled to a respective line replaceable unitdisposed in the aft section of the aircraft.

In the optical network depicted in FIG. 7, the output face of the 24×2transmit optical mixer 12 is optically coupled to the input face of the2×4 receive optical mixer 24 by fiber optical path 30, while the outputface of the 4×2 transmit optical mixer 22 is optically coupled to theinput face of the 2×24 receive optical mixer 14 by fiber optical path40. The fiber optical path 30 comprises six plastic optical fibers 32a-32 f optically coupled in series by five connectors 34 a-34 e, whilethe fiber optical path 40 comprises six plastic optical fibers 42 a-42 foptically coupled in series by five connectors 44 a-44 e. In addition,the output face of the 24×2 transmit optical mixer 12 is opticallycoupled to the input face of the 2×24 receive optical mixer 14 byplastic optical fibers 50 a and 50 b connected by a connector 52, whilethe output face of the 4×2 transmit optical mixer 22 is opticallycoupled to the input face of the 2×4 receive optical mixer 24 by plasticoptical fibers 54 a and 54 b connected by an optical attenuator 56.

A computer simulation determined that with the transmitter opticaloutput power and receiver sensitivity of the optical system shown inFIG. 7, taking into account POF optical loss and connector optical loss,an end-of-life optical link margin within the design target for a highlyreliable fiber optic system in an avionics environment could beachieved. However, a subsequent review of the installation representedby the configuration depicted in FIG. 7 indicated that the lengths ofthe fiber optical paths 30 and 40 should be increased.

To implement the proposed increase in length, it was determined that oneconnecter should be added to each of the fiber optical paths 30 and 40.The resulting configuration is depicted in FIG. 8, which is identical toFIG. 7 except that fiber optical path 30 has an additional (i.e., sixth)connector 34 f and an additional (i.e., seventh) plastic optical fiber32 g, and that fiber optical path 40 has an additional (i.e., sixth)connector 44 f and an additional (i.e., seventh) plastic optical fiber42 g. The plastic optical fiber 32 g runs from the connector 34 f to theinput face of the receive optical mixer 24; the plastic optical fiber 42g runs from the connector 44 f to the input face of the receive opticalmixer 14,

A computer simulation revealed that this increase in the number of POFlinks and connection breaks would reduce the end-of-line optical linkmargin of the system. Consequently, an effort was made to engineer astructural change that would increase the optical link margin to ahigher level. An analysis determined that the most reliable and robustapproach for achieving the desired improvement in the optical linkbudget would be to enhance the receiver sensitivity.

FIG. 9 is a diagram showing one end of a 1-mm-diameter output plasticoptical fiber 2 optically coupled to a 0.4-mm-diameter photodetector 64of a monolithic receiver IC chip 68 by way of a ball lens 62 inaccordance with one embodiment. The monolithic receiver IC chip 68 ispackaged inside a metal cap 60. The top of the metal cap 60 has acircular aperture in which ball lens 62 is seated. The ball lens 62 issoldered in place (see solder 66). The monolithic receiver IC chip 68has an integrated photodetector 64 in the form of a silicon PIN (p-typeintrinsic n-type) detector. The receiver IC chip 68 is configured tofunction as a burst mode receiver that generates electrical signalsbased on the optical signals detected by the photodetector 64.

The purpose of integrating the receiver electronics and thephotodetector 64 on the same chip is to reduce the size and maximize thesignal-to-noise ratio. Because of this size limitation, thephotodetector 64 in one commercially available receiver has a diameterof only 400 microns (0.4 mm). The ball lens 62 is the same commerciallyavailable receiver has a diameter of 2 mm. Coupling the 0.4-mm-diameterphotodetector 64 to the 1-mm-diameter output plastic optical fiber 2 asdepicted in FIG. 9 produces an optical coupling loss due to the mismatchin sizes. This optical coupling loss (OCL) can be calculated using thearea mismatch ratio: OCL=10×Log [(0.4/1)²] dB=−8 dB. This theoreticalcalculation shows an 8 dB optical loss in coupling each 1-mm-diameteroutput plastic optical fiber 2 to each 0.4-mm-diameter photodetector 64.

To compensate for the foregoing optical 8 dB coupling loss, the solutionproposed herein is to substitute output plastic optical fibers having adiameter which is less than 1 mm and preferably less than 0.4 mm.Experimental results in coupling smaller-diameter output plastic opticalfiber to a receiver having an integrated 0.4-mm-diameter photodetectorshowed an improvement in the receiver sensitivity. However, changing theoutput fiber size with the existing star coupler design depicted inFIGS. 10A and 10B was not feasible.

FIG. 10A shows a forward receive optical mixer 14 in the form of a 5-cmtapered glass mixing rod having two 1-mm-diameter input plastic opticalfibers 42 f and 50 b attached to a 2.5 mm×2.5 mm input face andtwenty-four 1-mm-diameter output plastic optical fibers 38 attached to a7 mm×7 mm output face. Nineteen of the plastic optical fibers 38 areoptically coupled to respective receivers (not shown in FIG. 10A) ofrespective optical electrical media converters located in the forwardsection of the aircraft. (When only nineteen of the twenty-four outputplastic optical fibers are needed, the extra five can be cut off.)

Similarly, FIG. 10B shows an aft receive optical mixer 24 in the form ofa 5-cm tapered glass mixing rod having two 1-mm-diameter input plasticoptical fibers 32 f and 54 b attached to a 2.5 mm×2.5 mm input face andfour 1-mm-diameter output plastic optical fibers 48 attached to a 5 mm×5mm output face. The output plastic optical fibers 48 are opticallycoupled to respective receivers (not shown) of respectiveoptical-electrical media converters located in the aft section of theaircraft.

The receive optical mixers 14 and 24 shown in FIGS. 10A and 10B aresymmetrical POF couplers having input and output faces connected to1-mm-diameter plastic optical fibers. This coupler design is veryadvantageous for the transmit optical mixers 12 and 22 (see FIG. 7) tomaximize the coupling of the transmitter laser output power to theoptical link. But for the receive optical mixers 14 and 24, each1-mm-diameter output plastic optical fiber has a large mismatch with theaforementioned 0.40-mm-diameter photodetector in the respectivereceiver. However, decreasing the size of the output plastic opticalfibers would create a large mismatch with the sizes of the output facesof the receive optical mixers 14 and 24.

The solution to this dilemma proposed herein is to design an asymmetricreceive optical mixer that allows the use of output plastic opticalfibers having a diameter less than the diameter of the input plasticoptical fibers. Various embodiments, in which input plastic opticalfibers have a diameter of 1 mm and output plastic optical fibers havevarious diameters less than 1 mm, will now be described. However, itshould be appreciated that the concept disclosed herein does not requireinput plastic optical fibers having a diameter of 1 mm andphotodetectors having a diameter of 0.4 mm. More generally, if thediameter d_(input) of each input plastic optical fiber is greater thanthe diameter d_(detector) of the photodetector, then the diameterd_(output) of each output plastic optical fiber should be less thand_(input) and preferably also equal to or less than d_(detector).

FIG. 11 is a diagram showing a forward receive optical mixer 100 of aforward star coupler in accordance with one embodiment. This forwardreceive optical mixer 100 comprises a 1-mm-diameter step-index plasticoptical fiber 102 having two D-shaped end faces of respective inputplastic optical fibers 104 and 106 attached to its input end face andnineteen 190-μm-diameter output plastic optical fibers 114 attached toits output end face.

FIG. 12 is a diagram representing an isometric view of two input plasticoptical fibers 104 and 106 converging to form a combiner 108 in theoptical mixer depicted in FIG. 11. FIG. 12A is a magnified view of theend portion of the combiner 108 depicted in FIG. 12. The end sections ofinput plastic optical fibers 104 and 106 are bonded together using alayer of index matching epoxy 105 to form the combiner 108. FIG. 12Bshows the semicircular end faces 120 and 122 of the plastic opticalfibers depicted in FIG. 12. In this embodiment, each of the end faces120 and 122 has a radius is equal to the radius (i.e., 0.5 mm) of the1-mm-diameter step-index plastic optical fiber 102 seen in FIG. 11.

Referring again to FIG. 11, the input plastic optical fibers 104 and 106comprise respective end sections (which end sections begin where thecircular cross sections of input plastic optical fibers 104 and 106transition to non-circular and terminate at the end faces 120 and 122)which are optically coupled and bonded to each other at an interface bythe layer of index matching epoxy 105. These optically coupled endsections form the combiner 108, which will be treated as being part ofthe forward receive optical mixer 100 (another part being theaforementioned 1-mm-diameter step-index plastic optical fiber 102). Theuse of the modifier “forward receiver” in the name “forward receiveoptical mixer” indicates that the 190-μm-diameter output plastic opticalfibers 114 (which each have one end optically coupled to the forwardreceive optical mixer) have other ends which are optically coupled toreceivers (not shown) located in the forward section of the aircraft.

As seen in FIGS. 11, 12, and 12A, the end section of input plasticoptical fiber 104 is shaped to form a first side face that intersectsand is perpendicular to end face 120, while the end section of inputplastic optical fiber 106 is shaped to form a second side face thatintersects and is perpendicular to end face 122. These side faces arethen bonded and optically coupled together by a layer of index matchingepoxy 105.

In accordance with one implementation of the embodiment depicted in FIG.11, the 1-mm-diameter step-index plastic optical fiber 102 is a standardhigh-temperature plastic optical fiber made of PMMA. The length LF ofthe 1-mm-diameter step-index plastic optical fiber 102 is preferably inthe range of 5 to 10 cm for uniform mixing of the input optical signalsduring their propagation therethrough. The two input plastic opticalfibers 104 and 106 have a diameter of 1 mm except in the respective endsections that form the combiner 108. The semicircular end faces 120 and122 seen in FIG. 12 are attached to the circular input end face of the1-mm-diameter step-index plastic optical fiber 102 seen in FIG. 11 usingindex matching epoxy. In accordance with one embodiment, each end face120 and 122 is a semicircle having a radius equal to the radius (i.e.,0.5 mm) of the circular input end face of the 1-mm-diameter step-indexplastic optical fiber 102.

In one implementation, the combiner 108 can have a length of about 8 mmor longer. The layer of index matching epoxy 105 (seen in FIG. 12) isused to bond confronting planar surfaces of the input plastic opticalfibers 104 and 106 together (without a metal layer) for the purpose ofenhancing the mixing uniformity of incoming two optical signals.

In accordance with an alternative embodiment, the combiner 108 may be aseparate monolithic optical transparent element having two circular1-mm-diameter input ends faces optically coupled to respective1-mm-diameter input plastic optical fibers and one circular1-mm-diameter output end face optically coupled to the input end face ofthe 1-mm-diameter step-index plastic optical fiber 102.

In accordance with one implementation of the embodiment depicted in FIG.11, a 1-mm-diameter multi-core plastic optical fiber bundle 110comprising nineteen 190-μm-diameter step-index plastic optical fibers114 bundled together is attached to the output end face of the1-mm-diameter step-index plastic optical fiber 102. The input face ofsuch a fiber bundle is shown in FIG. 13 (using circles that representideal, not real shapes of typical plastic optical fiber end faces). Thenineteen 190-μm-diameter output plastic optical fibers 114 are bundledtogether in an outer cladding 116. The 190-μm-diameter step-indexplastic optical fibers 114 of the 1-mm-diameter multi-core plasticoptical fiber bundle 110 are easily separated by dissolving the outercladding 116 of the fiber bundle using solvent or by mechanicallypeeling off the outer cladding 116.

FIG. 14 shows the true (not idealized) shapes of end faces of aplurality of 190-μm-diameter output plastic optical fibers of acommercially available multi-core plastic optical fiber bundle which canbe attached to the output end face of the 1-mm-diameter step-indexplastic optical fiber 102 depicted in FIG. 11. FIG. 15 is a diagramrepresenting a portion of such a separable 1-mm-diameter multi-coreplastic optical fiber bundle 110. This diagram shows the splayed ends ofthe nineteen 190-μm-diameter step-index plastic optical fibers 114 in asection where the outer cladding 116 has been removed.

Referring again to FIG. 11, a precision metal (or ceramic) ferrule 112(indicated by a dashed rectangle) having an inner diameter of 1 mm isused to house the entire 1-mm-diameter step-index plastic optical fiber102, a portion of the combiner 108, and a portion of the 1-mm-diametermulti-core plastic optical fiber bundle 110. (The 1-mm-diameterstep-index plastic optical fiber 102 and the combiner 108 when attachedto each other, form the forward receive optical mixer 100.) Additionalpotting optical epoxy is used to mount the 1-mm-diameter step-indexplastic optical fiber 102 and the aforementioned portions of combiner108 and 1-mm-diameter multi-core plastic optical fiber bundle 110securely inside the ferrule 112. The outer cladding of that portion ofthe 1-mm-diameter multi-core plastic optical fiber bundle 110 which isdisposed inside the ferrule 112 and not separated into individual fibersis not shown in FIG. 11.

An alternative to using the commercially available 1-mm-diametermulti-core plastic optical fiber 110 bundle is bundling nineteensmall-diameter plastic optical fibers (commercially availableindividually) into the ferrule 112, which has an inner diameter of 1 mm.The most suitable individual single-core small-diameter plastic opticalfiber is a 175-μm-diameter plastic optical fiber. FIG. 16 shows a fiberbundle 118 comprising nineteen 175-μm-diameter plastic optical fibers124 bundled together by an outer cladding 116. The outer diameter of theouter cladding 116 can be 1 mm to match the inner diameter of thesurrounding ferrule 112 shown in FIG. 11. Plastic optical fibers havinga diameter of 175 μm are commercially available from Asahi KaseiCorporation, Tokyo, Japan.

If more than nineteen channels in the forward receive optical mixer 100are needed, FIG. 17 shows that twenty-one individual 175-μm-diameterplastic optical fibers 128 can be fitted into a 1-mm ferrule. The endfaces of the twenty-one individual 175-μm-diameter plastic opticalfibers 128 can be attached to the output face of the 1-mm-diameterstep-index plastic optical fiber 102 depicted in FIG. 11. With this21-fiber option, the forward receive optical mixer 100 has theflexibility to have up to three spare channels. This will allow theforward star coupler to be implemented with 18 active receive channelsand three spare receive channels.

FIGS. 11 through 17 show a design and an implementation for anasymmetric forward receive optical mixer to be connected to a pluralityof receivers located in a forward section of an aircraft. This designincreases the link budget for optical signals propagating fromtransmitters located in the aft section of the aircraft. Similar changescan be made to the design and implementation of the aft receive opticalmixer.

FIG. 18 is a diagram showing an aft receive optical mixer 200 of an aftstar coupler in accordance with one embodiment. This receive opticalmixer 200 comprises a 1-mm-diameter step-index plastic optical fiber 202having two D-shaped end faces of respective input plastic optical fibers204 and 206 attached to its input end face and four 400-μm-diameteroutput plastic optical fibers 214 attached to its output end face. FIG.19 shows two D-shaped end faces 220 and 222 which are complementarysections of a circle having a diameter of 1 mm. The areas of the endfaces 220 and 222 meet along a chord of the circle, meaning that theconfronting surfaces are planar.

To minimize the area mismatch loss, the optimum diameter of the fouroutput plastic optical fibers 214 for use in aft receive optical mixer200 is 400 microns (0.4 mm). 400-μm-diameter plastic optical fiber isone of the standard sizes of individual single-core plastic opticalfiber that is commercially available. The 400-μm-diameter plasticoptical fibers 214 also have a good match to the diameter of thepreviously described photodetector 64 of the receiver shown in FIG. 9.

Referring again to FIG. 18, the input plastic optical fibers 204 and 206comprise respective end sections (which end sections begin where thecircular cross sections of input plastic optical fibers 204 and 206transition to non-circular and terminate at the end faces 220 and 222)which are optically coupled and bonded to each other at an interface bya layer of index matching epoxy 205. These optically coupled endsections form a combiner 208 which will be treated as being part of theaft receive optical mixer 200 (another part being the aforementioned1-mm-diameter step-index plastic optical fiber 202). The use of themodifier “aft receiver” in the name “aft receive optical mixer”indicates that the 400-μm-diameter output plastic optical fibers 214(which each have one end optically coupled to the aft receive opticalmixer) have other ends which are optically coupled to receivers (notshown) located in the aft section of the aircraft.

As seen in FIG. 18, the end section of input plastic optical fiber 204is shaped to form a first side face that intersects and is perpendicularto end face 220 (see FIG. 19), while the end section of input plasticoptical fiber 206 is shaped to form a second side face that intersectsand is perpendicular to end face 222 (see FIG. 19). These side faces arethen bonded and optically coupled together by means of index matchingepoxy 205.

In accordance with one implementation of the embodiment depicted in FIG.18, the 1-mm-diameter step-index plastic optical fiber 102 is a standardhigh-temperature plastic optical fiber made of PMMA. The length LA ofthe 1-mm-diameter step-index plastic optical fiber 202 is preferably inthe range of 5 to 10 cm for uniform mixing of the input optical signalsduring their propagation therethrough. The two input plastic opticalfibers 204 and 206 have a diameter of 1 mm except in the respective endsections that form the combiner 208. The end faces 220 and 222 seen inFIG. 19 are attached to the circular output end face of the1-mm-diameter step-index plastic optical fiber 202 seen in FIG. 18 usingindex matching epoxy. In accordance with one embodiment, each end face220 and 222 is a section of a circle having a diameter of 1 mm.

With the selection of 400-μm-diameter plastic optical fiber for couplingto the output end face of the 1-mm-diameter step-index plastic opticalfiber 202, an analysis was performed to determine whether the same 50/50splitting combiner as shown in FIG. 11 could be used to form combiner208. The analysis results indicated that the optical coupling loss washigher than a maximum allowable level.

As indicated by the internal architecture of the aft star coupler 20shown in FIG. 8, a left input arm (i.e., the plastic optical fiber 54 b)connects the aft receive optical mixer 24 to an optical attenuator 56,which in turn is connected to the aft transmit optical mixer 22 by aright output arm (i.e., the plastic optical fiber 54 a). This connectionis a local “wrap around” optical connection between the optical mixersinside the aft star coupler 20. Because of the high output power of theaft transmit optical mixer 22 and the low port count of the aft receiveoptical mixer 24, the optical attenuator 56 has a large attenuation.This provides an advantage of changing the splitting ratio of thecombiner 208 to achieve lower optical coupling loss in the aft starcoupler 20. By changing the combiner splitting ratio to 80/20 andlowering the attenuation of the optical attenuator 56, using plasticoptical fiber 206 as the left input arm 54 b of the aft receive opticalmixer 24, and using plastic optical fiber 204 as the right input arm 32g of the aft receive optical mixer 24, a total loss lower than themaximum allowable level can be attained.

FIG. 19 shows the asymmetric end faces 220 and 222 of the 1-mm-diameterinput plastic optical fibers 204 and 206. The end faces 220 and 222 areattached to the circular input end face of the 1-mm-diameter step-indexplastic optical fiber 202 and are attached to each other by the layer ofindex matching epoxy 205. When viewed from the end, the interface 205 isdefined by a chord that is located such that the ratio of the areas ofend faces 220 and 222 is 80/20.

FIG. 20 shows a fiber bundle 218 comprising four 400-μm-diameter plasticoptical fibers 214 bundled together using potting optical epoxy 216. Theouter diameter of the potting optical epoxy 216 is 1 mm to match theinner diameter of the surrounding ferrule 212 shown in FIG. 18. The endfaces of the four 400-μm-diameter output plastic optical fibers 214 areattached to the output end face of the 1-mm-diameter step-index plasticoptical fiber 202 depicted in FIG. 18.

Further increase in the splitting ratio of the combiner 208 to 90/10would further reduce the optical coupling loss of the aft receiveoptical mixer 200. However, for splitting ratios larger than 80/20, theprocess for manufacturing the combiner becomes more difficult.

In summary, this disclosure has presented asymmetric plastic opticalfiber star coupler designs that can be incorporated in an avionicssystem to increase the end-of-life optical link margin of the opticaldata bus, reducing the installation and maintenance cost and increasingthe reliability of the system.

In accordance with one embodiment, an optical mixer of the type depictedin FIG. 11 can be installed in an optical network using a methodcomprising the following steps: cutting a length of a first plasticoptical fiber 102 to form first and second end faces, each of the firstand second end faces having a first area; shaping an end section of asecond plastic optical fiber 104 having a cross-sectional area equal tothe first area to form a first side face that intersects and isperpendicular to a first end face 120 having a second area which is lessthan the first area; shaping an end section of a third plastic opticalfiber 106 having a cross-sectional area equal to the first area to forma second side face that intersects and is perpendicular to a second endface 122 having a third area which is less than the first area; bondingthe first and second side faces of the second and third plastic opticalfibers together using index matching epoxy 105; bonding the first andsecond end faces 120 and 122 of the second and third plastic opticalfibers 104 and 106 to respective portions of the first end face of thelength of the first plastic optical fiber 102 using index matchingepoxy; bonding the end faces of a plurality of plastic optical fibers114, each having a fourth area less than the first area, to respectiveportions of the second end face of the length of the first plasticoptical fiber 102 using index matching epoxy; securing the length of thefirst plastic optical fiber 102, mixer 108 and cladding 110 inside aferrule 112 using potting optical epoxy; and connecting the second andthird plastic optical fibers 104 and 106 and the plurality of plasticoptical fibers 114 to respective other components of the opticalnetwork. In the disclosed embodiments, the sum of the second and thirdareas is equal to the first area.

While optical networking systems have been described with reference tovarious embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the teachings herein. Inaddition, many modifications may be made to adapt the concepts andreductions to practice disclosed herein to a particular situation.Accordingly, it is intended that the subject matter covered by theclaims not be limited to the disclosed embodiments.

The invention claimed is:
 1. A fiber optical system comprising: a mixingoptical fiber having a first end face with a first area and a second endface with the first area; a first input plastic optical fiber having afirst end face with the first area and a second end face with a secondarea less than the first area, the second end face of the first inputplastic optical fiber being attached to a first portion of the first endface of the mixing optical fiber; a second input plastic optical fiberhaving a first end face with the first area and a second end face with athird area less than the first area, the second end face of the secondinput plastic optical fiber being attached to a second portion of thefirst end face of the mixing optical fiber; and a plurality of outputplastic optical fibers having end faces with a fourth area less than thefirst area attached to the second end face of the mixing optical fiber.2. The fiber optical system as recited in claim 1, wherein the sum ofthe second and third areas is equal to the first area.
 3. The fiberoptical system as recited in claim 2, wherein the first input plasticoptical fiber has a first side face that intersects the second end faceof the first input plastic optical fiber, and the second input plasticoptical fiber has a second side face that intersects the second end faceof the second input plastic optical fiber, further comprising a layer ofindex matching epoxy disposed between and bonding confronting portionsof the first and second side faces with no metal layer therebetween. 4.The fiber optical system as recited in claim 1, wherein the second areais equal to the third area.
 5. The fiber optical system as recited inclaim 1, wherein the first and second end faces of the mixing opticalfiber have a circular shape.
 6. The fiber optical system as recited inclaim 5, wherein the second end faces of the first and second inputplastic optical fibers are disposed adjacent to each other and formrespective sections of a circle that meet along a chord of the circle.7. The fiber optical system as recited in claim 1, wherein the mixingoptical fiber comprises a step-index plastic optical fiber.
 8. The fiberoptical system as recited in claim 1, further comprising an outercladding in which respective first sections of the plurality of outputplastic optical fibers are embedded, wherein each of the output plasticoptical fibers comprise a respective second section extending beyond theouter cladding, wherein the second sections of the plurality of outputplastic optical fibers are splayed.
 9. The fiber optical system asrecited in claim 8, further comprising: a ferrule that surrounds themixing optical fiber, portions of the first and second input plasticoptical fibers adjacent to the mixing optical fiber, and at least aportion of the outer cladding; and potting optical epoxy disposed insidethe ferrule for securing the mixing optical fiber, portions of the firstand second input plastic optical fibers adjacent to the mixing opticalfiber, and at least a portion of the outer cladding inside the ferrule.10. The fiber optical system as recited in claim 1, further comprising:a tapered mixing rod comprising an input face having a fifth area and anoutput face having a sixth area, the fifth area being greater than thesixth area; and a plurality of third input plastic optical fibers havingend faces with the first area attached to the input face of the taperedmixing rod, wherein the first end face of the first input plasticoptical fiber is attached to the output face of the tapered mixing rod.